Gibbs free energy change and stability of chromium valence transition reactions in magnesium chromate

Abstract Chromium oxide is present in the oxidation slag generated during the smelting of Cr‐containing steel in an electric furnace. A prior study reported that chromium oxide is primarily present as MgCr 2 O 4 phase in the oxidation slag. However, when the trivalent chromium is oxidized during slag treatment to produce harmful hexavalent chromium (MgCrO 4 phase), the slag is rendered unsuitable for use as a road construction material. A previous study showed that the chemical reaction between MgCrO 4 and MgCr 2 O 4 is MgCrO 4 (s) = 1/2MgCr 2 O 4 (s) + 1/2MgO(s) + 3/4O 2 (g). The free energy change of this reaction, which was not obtained, is essential to determine the thermal conditions required to suppress the formation of the MgCrO 4 phase. In this study, by performing thermal analyses, such as thermogravimetry and differential scanning calorimetry on MgCrO 4 , thermodynamic data on the decomposition reaction of MgCrO 4 were obtained. Consequently, ∆ G ° = 63 000 − 75.8 T (J/mol) for the aforementioned reaction and ∆ G ° = −69 900 − 44.8 T (J/mol) for the reaction MgO(s) + CrO 3 (s) = MgCrO 4 (s) were derived.